Dive 04: Sibelius Seamount and Midwater Transects
September 10, 2017
Access Dive Summary and ROV Data

Today’s dive was the best of both worlds: exploration of both the seafloor and the water column. For the benthic portion of the dive, remotely operated vehicle (ROV) Deep Discoverer (D2) reached the seafloor of Sibelius Seamount at a depth 2,600 meters (8,530 feet), slowly climbed up the steep slope of a circular outcrop, and finally moved up the ridge to the flat summit of the seamount. Early on in the dive, the team observed a talus (broken rocks) slope with a relatively low density of animals, including jellyfish, a tumbling snail, sea cucumbers, and crinoids. As D2 moved up the slope, the density of animals greatly increased and we observed an ensemble of benthic organisms, including a variety of corals, sponges, echinoderms, sea stars, fishes like sorceress and cusk eels, and more. In total, the team observed two separate high-density communities.

After the benthic portion of the dive, the ROV pilots maneuvered D2 up through the water column and conducted 100-meter midwater transects at depths 800, 700, 600, 500, 400, and 300 meters. During the ascents between transect depths, the ROV tether was kept behind the vehicle so as not to disturb the water, and the ROVs ascended slowly at a speed of about 10 meters per minute. While exploring at various depths in the water column, the team observed siphonophores, ctenophores, salps, jellies, an eel, a hatchetfish and other fishes, and pteropods. D2 also got inked by a squid! This was the first of seven dives during this expedition that will be wholly or partially dedicated to water column exploration. The team is excited to continue exploring this largely unknown biome.